Hiraga Gennai was a Japanese polymath and rōnin of the Edo period. He was a pharmacologist, student of Rangaku, physician, author, painter and inventor well known for his Erekiteru, Kandankei (thermometer) and Kakanpu. Gennai composed several works of literature, including the fictional satires Fūryū Shidōken den (1763), the Nenashigusa (1763), and the Nenashigusa kohen (1768), and the satirical essays On Farting and A Lousy Journey of Love. He also authored two guidebooks on the male prostitutes of Japan, the Kiku no en (1764) and the San no asa (1768). His birth name was Shiraishi Kunitomo, but he later used numerous pen names, including Kyūkei (鳩渓), Fūrai Sanjin (風来山人), Tenjiku rōnin (天竺浪人) and Fukuchi Kigai (福内鬼外). He is best known by the name Hiraga Gennai.

Rangaku, and by extension Yōgaku , is a body of knowledge developed by Japan through its contacts with the Dutch enclave of Dejima, which allowed Japan to keep abreast of Western technology and medicine in the period when the country was closed to foreigners from 1641 to 1853 because of the Tokugawa shogunate's policy of national isolation (sakoku).